This invention relates to the field of styrene manufacture and more particularly discloses apparatus including reactor vessels for the dehydrogenation of ethylbenzene into styrene monomer.
It is well known in the art of styrene manufacture to react ethylbenzene over a dehydrogenation catalyst such as iron oxide under elevated temperatures in the range of around 1000° F. and at a pressure of about 10 to 20 PSIA in order to strip hydrogen from the ethyl radical on the benzene ring to form the styrene molecule. This is normally done in a styrene radial reactor which also is commonly termed an EB dehydro reactor. The dehydro reactors generally are elongated cylindrical vertical structures of a very large size ranging in diameter from about five to thirty feet or more and in length from about ten to one hundred feet or more. The normal construction for such a reactor allows for input of the ethylbenzene gas at an inlet located in the bottom center of the vertical reactor, whereupon the gas is flowed up through an annular area, passing radially outward through a porous catalyst bed of iron oxide or other suitable dehydro catalyst, and then passing upward through an outer annular area to exit at the top of the reactor shell. Since the flow of ethylbenzene across the catalyst bed is in a radial direction, these reactors are sometimes identified as “radial” reactors.
Normally a radial reactor would be sized such that the annular flow area inside the catalyst bed would have some relative proportional value with respect to the cross-sectional flow area of the inlet pipe delivering ethylbenzene to the reactor. Preferably the annular flow area inside the catalyst bed would be larger than the cross-sectional flow area of the flow inlet pipe. Because of the extended vertical length of such reactors, normally the inlet pipe to the bottom of the reactor must come in at a relatively sharp ninety-degree radius and the resulting effect is a side-to-side maldistribution of flow across the reactor vessel. Ideally, the inlet pipe to the reactor would be a straight vertical pipe for a considerable distance prior to entering the reactor, but due to physical configurations, this is not possible because of the extended vertical height of the reactor.
Also, due to the nature of flow across the extended vertical length of the reactors, switching from longitudinal or axial flow into radial or transverse flow and then back into longitudinal flow, flow velocities across the catalyst bed from top to bottom vary widely in conventional reactor vessels, thus resulting in degraded catalyst life in those areas of the reactor with the greatest flow velocities. It has been found by experimentation and flow velocity measurements that the highest feed velocity across the catalyst beds in a radial reactor generally occurs near the top of the reactor, and the lowest velocity across the catalyst bed occurs near the bottom of the reactor near the inlet pipe. This increased velocity at the top of the catalyst bed and reduced velocity at the bottom of the catalyst bed results in a greatly shortened life of the catalyst near the top of the reactor and forces a shutdown of the reactor for catalyst regeneration much sooner than normally desirable.
Accordingly, it is desirable to improve the flow in the reactor both in the axial and vertical directions. U.S. Pat. No. 5,358,698 to Butler et al. issued on Oct. 25, 1994 and is assigned to Fina Technology, Inc. This '698 patent discloses a method for improving the flow in a dehydrogenation reactor by using a displacement cylinder. The disclosure of this patent is hereby incorporated by reference in its entirety. While improvement in fluid flow is achieved by the method taught in the '698 patent, further improvements were needed in order to improve the efficiency of the catalyst.